# Is it economically reasonable to cool the roof by water evaporation

I want to estimate the outcome before going into hassle of installing the sprinklers/mist emitters.

I have a place on the last floor, you can call it an attic, directly under a flat roof which is metal, insulated underneath with 15 cm of glass wool. 160 square meters of roof that is.

Most of the year temperatures in Kiev are below 28 °C, and I have decent ventilation, so the temperature inside is not more than 1 degree hotter than outside with full occupancy of 50 people. But those few hours in few days when the temperature rises above 30 °C is a problem for me.

Online calculators tell me that the heat is coming in mostly from sun radiation through windows and roof, and I need 19kW of cooling power to compensate for it.

I want to avoid installing air conditioning both because of up-front cost of more than 1000\$, and operating cost of electricity to tun it.

The numbers below are in hryvnas, exchange rate is roughly 20 hryvnas per 1 US dollar. Ballpark calculation is very promising: with a cost of water supply at around 10 ₴/cubic meters, evaporation heat cost of water would be 0.02 ₴/kWh, which is 2 orders of magnitude less than cost of electricity which is around 2 ₴/kWh. Estimated water usage would be 100 liters/hour, which is reasonable.

The question is -- how much of that cooling power would work for me cooling my space through 15 cm of glass wool between my ceiling and roof surface. That is versus cooling ambient air just above the roof. And how much water can I expect to evaporate on 160 sq.meters of roof at 30 °C, humidity rarely rises above 40% at such temperatures in Kiev.

Is a roof sprinker a good idea?
Would putting a sprinkler on my roof help cool my home?

• For what it's worth: one of the fringe benefits of installing rooftop solar panels has been that they now absorb and/or re-radiate a lot of the sunlight that was previously heating my attic. Jun 9, 2015 at 0:59
• Why not put a biodiverse or green roof on?
– user39204
Jul 7, 2015 at 9:56

Won't work. The insulation is sufficient to reduce the heat loss from your apartment at 25c to the snow or ice at -15c to a reasonably low value.

Misters or evaporative coolers only reduce the temperature by a few degrees C, and most of the heat absorbed would come from the surrounding air, not your house.

I'd suggest awnings over the windows, drapes/shades, and an attic fan, and possibly an indoor mister.

• +1 for the attic fan suggestion. This fairly inexpensive and can make a huge difference! Jun 8, 2015 at 20:21
• This reasonably low heat loss sums up to monthly electricity bill averaging 10 Kw in spring month. Would love to have those 10kW as a cooling power available for me. Drapes are already in place, most of the windows are luckily facing north. The place i'm trying to cool is the attic itself, I have fans totaling at 9000 cubic meters/hour air exchange.
– Gleb
Jun 9, 2015 at 10:37
• Paint the roof white? Cost of white roof paint (required in NYC) is pretty low. I just have a hard time believing that any significant fraction of the heat absorbed in evaporating the water is going to come from the attic-- in my house the top floor gets much hotter because the hot air from lower floors rises. Running a mister outside will just make the air outside a little cooler. Jun 9, 2015 at 15:15
• I agree with everything except the attic fan. They have been proved out to be only have any uses in very specific applications. Almost every green study I have read has pointed out that the cost of operating the fan highly outweighs its return. Jul 7, 2015 at 17:27
• Kind of depends how long you run them -- if your attic is 20 degrees hotter than outside you can run them for an hour and eliminate a heat source in the house. Jul 7, 2015 at 17:40

Before investing in active elements (electrical pumps), why not try out a passive installation? Simply building a second roof above the first -leaving an air-gap between the two- will help reduce incoming heat. The higher roof doesn't even need to be very strong, since it only has to handle its own weight (and perhaps also some wind).

This used to be a feature of classical Land Rovers used in tropical countries, called the "safari roof". Also in the Spanish region of Andalucia. No aircon needed - just keeping the direct sun away from the roof the occupants were under helped get temperatures down some 5-10 degrees C. You can see a picture here: http://landrover-4x4.co.uk/gawain/ldy04/safari_files/index.php (not my car, although I wish it was).

HTH.

• Was considering that. But the roof surface is sheet metal, that's 70% of radiation reflected already. And upfront cost of building the shading roof is huge.
– Gleb
Jun 9, 2015 at 10:45
• You might want to check this; this table suggests otherwise. deansteelbuildings.com/products/panels/sr-sri-by-color Jun 9, 2015 at 15:17